Interference technique and apparatus for spectrum analysis

ABSTRACT

4. SPECTRUM ANALYSIS APPARATUS COMPRISING: AN OPTICAL INTERFERMETER INCLUDING A RECIPROCABLE MIRROR AND HAVING A LIGHT EXIT PATH, MEANS FOR RECIPROCATING SAID MIRROR WITH AN EXCURSION EXCEEDING THE LONGEST WAVELENGTH OF THE LIGHT IN SAID EXIT PATH AND WITH A SUBSTANTIALLY CONSTAND VELOCITY OVER (AN APPRECIABLE) A MAJOR PORTION OF EACH RECIPROCATING CYCLE, PHOTECLECTRIC PICK-UP MEANS LOCATED IN THE LIGHT EXIT PATH OF SAID INTERFORMER FOR GENERATING A SIGNAL PROPORTIONAL TO THE OUT PUT IN SAID EXIT PATH, AND ELECTRONIC FREQUENCY ANALYZER MEANS COUPLED TO AND SUPPLIED WITH SAID SIGNAL, FOR MEASURING THE AMPLITUDES OF THE VARIOUS COMPONENT FREQUENCIES MAKING UP TO THE OUTPUT SIGNAL FROM SAID PHOTOELECTRIC PICKUP MEANS.   WHEREBY THE DISTRIBUTION OF THE AMPLITUDE OF THE SIGNAL AMONG ITS VARIOUS FREQUENCY COMPONENTS INDICATES THE DISTRIBUTION OF THE OPTICAL ENERGY OF THE GIVEN LIGHT SOURCE AMONG THE VARIOUS CORRESPONDING WAVELENGTHS.

March 26, 1974 MERTZ Re. 27,947

INTERFERENCE TECHNIQUE AND APPARATUS FOR SPECTRUM ANALYSIS OriginalFiled June 29, 1961 United States Patent Ofl'icc Reissued Mar. 26, 197427,947 INTERFERENCE TECHNIQUE AND APPARATUS FOR SPECTRUM ANALYSISLawrence N. Mertz, Lexington, Mass., by Block Engineering, Inc.,Cambridge, Mass., assignee Original No. 3,286,582, dated Nov. 22, 1966,Ser. No.

120,600, June 29, 1961. Application for reissue Aug.

16, 1971, Ser. No. 172,330

Int. Cl. Glb 9/02 US. Cl. 356-106 S 19 Claims Matter enclosed in heavybrackets II] appears in the original patent but forms no part of thisreissue specification; matter printed ln italics indicates the additionsmade by reissue.

The field of this invention is that of optical spectrum analysis moreparticularly to the use of interference techniques for this purpose.

Interferometry as applied to spectroscopy in general possesses severaldistinct advantages over conventional dispersive techniques, such asthose using prisms and gratings, for obtaining information as to energydistribution within the spectrum. Principal among these advantages aregreatly increased sensitivity, since light from the source underexamination is not scattered, and increased resolution. Heretofore,however, interferometry has not been particularly favored as a tool forspectral analysis because of certain intrinsic difficulties in datareduction. Primarily, these difficulties flow from the fact that thebrightness or intensity function obtained directly from the operation ofan interferometer is not a representation of the source energydistribution but is rather a representation of a function, the inverseFourier transform of which represents that energy distribution.

Previously, special analysis by interferometric techniques was carriedout by slowly changing the relative lengths of interfering light paths,for example by displacing a mirror in a conventional Michelsoninterferometer, and measuring the intensity of the interference fringesthereby produced as a function of path difference. This function,obtained in the form of a single interferogram, was then transformedeither by laborious hand calculation or by programming a relativelyelaborate computer, to yield the energy profile. A chopper has to beused in the light path in order to obtain an AC. signal that can bepractically handled.

Objects of the present inventions are to provide a technique of spectralanalysis by interferometry which permits the obtaining of an energydistribution profile without the above mentioned difficulties especiallyextensive calculation, and which yields high sensitivity and highresolution with a minimum of effort but high accuracy for all purposesincluding the gathering of spatial information and the observation ofrapidly fluctuating sources; to provide apparatus for spectral analysiswhich is simple and rapid in operation, which is of compact and lightweight construction, which is relatively inexpensive, which will operatewith all types of, including weak or unsteady light sources, which ishighly reliable and which does not require an elaborate or expensiveoptical system.

The substance and nature of the invention can be summarized incharacteristic aspect as follows.

Light from a source to be examined is passed through an interferometerthat permits the varying of the difference between the two interferinglight paths within the interferometer [for example] by means of amovable mirror. By doing this at a constant transposition velocity, afluctuating light output can be obtained which is a linearrepresentation of the variation in time of the intensity of fringes, asopposed to the displacement or path difference function mentioned abovewith reference to conventional spectroscopy. This time function ischaracteristically made up of component frequencies, the amplitudes ofwhich are representative of the energy distribution of the source lightamong corresponding wavelengths. The correspondence between thecomponent frequencies in the light output and the wavelengths of thesource light is dependent upon the particular transposition velocity atwhich the path dilference is varied, such as by moving a mirror or otheroptical instrumentality.

By choosing the transposition velocity so that the component frequenciesare within the range of frequencies that can be conveniently handled byconventional electronic equipment as, for example, the audio frequencyrange, and by repeating the movement such as that of a mirror, an outputsignal can be obtained by way of a photoelectric detector, and analyzedwith a conventional electronic frequency analyzer. The frequenciesinvolved are such that optical chopping for producing an alternatingcurrent signal is unnecessary. The spectrum obtained in terms of audiofrequencies is then an accurate representation of the optical spectrumdesired the correspondence of these audio frequencies with lightwavelengths being determined by the transposition velocity used.

In a practically important embodiment of the invention the mirror of aMichelson type interference device is driven [sawtooth fashion] with asubstantially constant speed scan and a rapid return by anelectrodynamic motor and the output signal from the photoelectric pickupis first recorded on magnetic tape. A portion of the tape, typically alength containing the record of approximately twenty sweeps of themirror, is then spliced into a loop and the loop is played back into ahererodyne frequency analyzer. This procedure allows the analysis to beperformed at leisure without the necessity of maintaining the lightsource. Likewise, the availability of a continuous source of thecomponent frequencies permits the electronic frequency analyzer to yieldmaximum resolution.

Apparatus according to a typical embodiment of the invention thusinvolves an interferometer having a reciprocal mirror and [photoelectricpickup] detector means in its light exit path, means for repetitivelyreciprocating the mirror in such manner that the velocity of the mirroris substantially constant for [an appreciable] a major portion of eachcycle, and electronic frequency analyzer means.

These and other objects, and aspects of the substance and nature of theinvention will appear from the herein presented outline of its principleand general mode of operation together wth a detailed description ofpractical embodiments illustrating its novel characteristics.

The description refers to a drawing in which FIG. 1 is a schematicrepresentation of interference spectrographical apparatus according tothe invention;

FIG. 2 illustrates, similarly to FIG. 1, a preferred embodiment of theinvention incorporating a tape recorder; and

FIGS. 3 and 4 are diagrams illustrating [the method according to] theinvention.

FIG. 1 indicates a lens system 10 arranged to direct an extended fieldof light into an interferometer 20 in its general construction ofconventional Michelson type. The interferometer has a beam splitter 22tilted at an angle of 45 with respect to the path of the incident light.Mounted parallel to the path of the incident light and to one side ofthe beam splitter 22 is a fixed mirror 24. Behind the beam splitter 22and facing the lens system 10 is a reciprocable mirror 26. The lightexit aperture of the interferometer is indicated at 29- on the oppositeside of the beam splitter 22 from the fixed mirror 24, and aphotoelectric detecting device such as phototube 30 is placed in theexit path.

The reciprocable mirror 26 is precision mounted for movement by anelectrodynamic motor 27 which is similar to the voice coil and magnetstructure used in conventional cone-type loud speakers, an appropriatelycompliant suspension being provided for the mirror and coil. For drivingthe mirror there is provided for the mirror and coil. For driving themirror there is provided an electronic power amplifier 34 controlled bya conventional sawtooth oscillator 36 preferably providing a constant velocity scan motion and a faster return motion. By this means theslidable mirror 26 can be reciprocated in such a manner that thevelocity of the mirror is substantially constant during the majorportion of each cycle. In particular, the velocity will be substantiallyconstant at that point in the mirrors travel which yields equal opticalpath lengths.

The electrical signal output of the photoelectric pickup 30 is amplifiedas indicated at 40, to a level suitable for analysis by [a] conventionalelectronic frequency [analyzer] analysis means indicated at 44.

The operation of the above described device is as follows. Light comingfrom the lens system and incident on the beam splitter 22 is dividedsubstantially equal between two paths one of which is deflected to thefixed mirror 24 and the other of which continues, in the same directionas the incident light, to the reciprocable mirror 26. In Well-knownfashion each of these half-beams is reflected by its respective mirrorback to the beam splitter 22 where they interfere, and a portion of thetotal available light, the amount depending upon the relative pathlengths, is directed into the photoelectric pickup 30. The fielddirected into the photocell corresponds to the central fringe of acircular interference pattern. As the mirror 26 reciprocates, the lightemerging from the interferometer fluctuates according to a time-energyfunction, the amplitude of whose component frequencies are, as explainedpreviously, representative of the wavelength distribution of theincident light. In accordance with the invention, these components areselected by the proper choice of velocity for the reciprocating mirror26, to fall within the range of frequencies conveniently handled byconventional electronic equipment. Accordingly, the composite electricaloutput signal from the photoelectric pickup 30 can be analyzed byconventional frequency analyzing apparatus 44. For analyzing the usualvisual or infrared bands, mirror 26 travels such as 0.004 inch and 0.018inch with scanning rates of 0.5 to 4 scans per second have beensuccessfully used. It will be understood that these will depend greatlyon the particular wavelengths and desired resolutions involved, thelatter depending on the length of the mirror travel. The informationobtained from the analyzer 44 as to the relative amplitudes of thevarious frequency components is directly representative of theamplitudes of the various wavelength components of the incident light.

Spectrographic signals obtained from this interferometer as illustrateddiagrammatically in FIGS. 3 and 4. If the incident light were trulymonochromatic the output signal obtained from the photoelectric pick-up30 would be sinusoidal over the entire constant velocity portion of thereciprocating mirrors travel as shown in FIG. 3. A typical use for thisapparatus, however, is the derivation of the energy profile of arelatively broad band source and a somewhat simplified illustration ofthe signal obtained in such a case is shown in FIG. 4. This signal issymmetrical about that portion of its amplitude which corresponds tozero path difference between the portions of the incident lightreflected from the fixed mirror 24 and from the reciprocable mirror 26.This signal is not a sine wave but rather the amplitudes of thefluctuations decrease with increasing path differences. This is becausethe output signal is a complex wave made up of many component fr quecies corresponding to the various wavelength components present in theincident light, and these various frequencies are in phase only for zeropath difference. The various components become more and moreout-of-phase as the path difference increases in either direction untilthe cancellation becomes virtually complete and thus the visibility ofthe interference fringes and the amplitude of the corresponding voltagesignal fluctuations also decrease.

Accordingly, if the sweep of the reciprocable mirror 26 passes farenough to either side of the point which corresponds to zero pathdifference, the fluctuations in the output signal from the photoelectricpickup 30 will drop to practically zero. Because of this feature, theperiod during which the mirror reverses direction and hence is nottraveling at constant velocity has no appreciable effect on theamplitudes of the various frequency components of the output signal.

In a satisfactory working arrangement, the travel of the mirror takesplace in conformity to a saw-tooth function, the return speed beingabout ten times as fast as the scanning speed. In many instances thehigh frequencies introduced into the output signal by the return travelwill be outside the frequency range of the analyzer and can hence bedisregarded. If a very large optical band width has to be covered, thereturn stroke can be blanked by conventional electronic apparatus. As analternative, since the output signal is symmetrical around the pointwhich represents zero path difference, the output signal produced duringthe sweep of the mirror in both directions can be utilized therebyavoiding any problems in switching.

FIG. 2 illustrates a modified, preferred embodiment of the invention inwhich a tape recorder 42 is interposed between the photoelectric pickup30 and the electronic frequency analyzer 44. As pointed out above, ifthe output signal from the photoelectric pickup is recorded on magnetictape and a portion of the recording is spliced into a loop, analysis ofthe component frequencies can be performed at leisure and convenience,and over a longer time than the light source can be maintained. Also,the range of frequencies presented to the analyzer can be expanded orcondensed by playing the tape back at a different speed from that atwhich it was recorded. A furthat control of this range of frequenciescan be obtained by varying the velocity at which the reciprocatingmirror 26 makes its sweeps. This transposition velocity control can beaccomplished by adjusting the frequency produced by the saw-toothoscillator 36.

While the tape recorder 42 is indicated as a single unit it should beunderstood that separate recording and play back facilities can be used.This is especially true in the preferred practice of the invention inwhich the tape is transformed into a loop before being played back intothe electronic frequency analyzer 44. In a typical embodiment, a loop of15 to 20 interferograms constitutes a convenient size and the noisegenerated by the tape splice is not objectionable with that number ofinterferograms.

Panoramic wave analyzers can be used either operated directly from theinterferometer or from the tape recorder. In this manner graphs of thespectrum can be immediately produced. If only particular wavelengths areof interest, such can be separated by means of electrical band passfilters.

Because this device does not depend upon the use of narrow slits toobtain high resolution but has an effective aperture equal to the sizeof the smallest mirror, its sensitivity is correspondingly greater thanthat of conventional, dispersive types of spectrometers using prisms orgratings. An increase in sensitivity by a factor of one thousand can beeasily obtained. A further advantage of this device over comparablyelaborate instruments of the dispersive type is its increasedresolution. This advantage flows from the nature of the interferometricprocess in which the phenomenon detected is not a function of a singlewavelength but rather a function to which all of the wavelengthcomponents contribute simultaneously; in other words, all of thewavelengths are measured at the same time.

The simultaneous measurement of all wavelengths which is characteristicof the technique according to the invention leads to two furtheradvantages. The first is that, if the light source fluctuates inintensity even more rapidly than the mirror oscillates, the energyprofile or spectrum obtained will still be of the proper relative shapeand only the fine resolution will be affected. Secondly, thesignal-to-noise ratio is improved because the time over which eachwavelength is measured is much greater than in equipment in which eachwavelength is measured separately.

It should be understood that the present disclosure is for the purposeof illustration only and that this invention includes all modificationsand equivalents which fall within the scope of the appended claims.

I claim:

[1. The method of optical spectrum analysis with an interefrometerhaving a reciprocable mirror and having photoelectric detecting means inits light exit path, comprising the steps:

directing light within an appreciable spectral range of opticalfrequencies from a source to be examined through the interferometer;reciprocating the mirror with substantially constant velocity over anappreciable portion of each reciprocating cycle, at an excursion, and ata repetition rate to produce fluctuating light amplitude outputs atfrequencies related to the component optical frequencies contained inthe source of light to be examined;

directing said fluctuating light amplitude outputs on to saidphotoelectric detecting means; and

analyzing the amplitude of each frequency component of the output signalobtained from said photoelectric detecting means during that portion ofeach reciprocating cycle during which the velocity of the mirror issubstantially constant to provide an indication of the intensities ofthe various frequency components within the spectral range of saidsource, and indicating the distribution of the optical energy of saidlight source among the various corresponding wavelengths correspondingto said various frequency components] [2. The method of optical spectrumanalysis with an interferometer having a reciprocable mirror and havingphotoelectric detecting means in its light exit path, comprising thesteps:

directing light within an appreciable spectral range of opticalfrequencies from a source to be examined through the interferometer;

reciprocating the mirror with a substantially constant velocity over anappreciable portion of each reciprocating cycle, at an excursion, and ata repetition rate to produce fluctuating light amplitude outputs atfrequencies related to the component optical frequencies contained inthe source of light to be examined;

directing said fluctuating light amplitude outputs on to saidphotoelectric detecting means;

recording the output signal from said photoelectric detecting meansduring that portion of each reciprocating cycle during which thevelocity of the mirror is substantially constant; and

playing back repetitively the signal so recorded into frequency analyzermeans to provide an indication of the intensities of the variousfrequency components within the spectral range of said source, andindicating the distribution of optical energy among the variouscorresponding wavelengths] [3. The method of optical spectrum analysiswith an interferometer having a reciprocable mirror and havingphotoelectric detecting means in its light exit path, comprising thesteps:

directing light within an appreciable spectral range of opticalfrequencies from a source to be examined through the interferometer;

reciprocating the mirror with a substantially constant velocity over anappreciable portion of each reciprocating cycle, at an excursion, and ata repetition rate to produce fluctuating light amplitude outputs atfrequencies related to the component optical frequencies contained inthe source of light to be examined;

directing said fluctuating light amplitude outputs on to saidphotoelectric detecting means;

recording on a tape a representation of the electrical output signalfrom said photoelectric pick-up during that portion of eachreciprocating cycle during which the velocity of the mirror issubstantially constant;

splicing said tape into a loop; and

playing back said tape recording loop into a frequency analyzer toprovide an indication of the intensities of the various frequencycomponents within the spectral range of said source indicating thedistribution of optical energy among the various correspondingwavelengths] 4. Spectrum analysis apparatus comprising:

an optical interferometer including a reciprocable mirror and having alight exit path;

means for reciprocating said mirror with an excursion exceeding thelongest wavelength of the light in said exit path and with asubstantially constant velocity over [an appreciable] a major portion ofeach reciprocating cycle;

photoelectric pick-up means located in the light exit path of saidinterferometer for generating a signal proportional to the output insaid exit path; and

electronic frequency analyzer means coupled to and supplied with saidsignal, for measuring the amplitudes of the various componentfrequencies making up the output signal from said photoelectric pickupmeans;

whereby the distribution of the amplitude of the signal among itsvarious frequency components indicates the distribution of the opticalenergy of the given light source among the various correspondingwavelengths.

5. Spectrum analysis apparatus comprising:

an optical interferometer including a reciprocable mirror and having alight exit path;

means for reciprocating said mirror with an excursion exceeding thelongest wavelength of the light in said exit path and with asubstantially constant velocity over [an appreciable] a major portion ofeach reciprocating cycle;

photoelectric pick-up means located in the light exit path of saidinterferometer for generating a signal proportional to the output insaid exit path;

electronic recording means connected to said pick-up means and suppliedwith said signal, said recording means including record carrying meansand playback means; and

frequency analyzer means coupled to and receiving signals derived fromsaid record carrying means, for measuring the amplitudes of the variouscomponent frequencies making up the recorded output signal from saidphotoelectric pick-up means thereby to provide an indication of theamplitude distribution of the signal among its various frequencycomponents which distribution indicates the distribution of the opticalenergy of a given light source among its various correspondingwavelengths.

6. Spectrum analysis apparatus comprising a Michelson interferometerincluding a contimrously reciprocable mirror and having a light exitpath;

means for continuously reciprocating said mirror with an excursionexceeding the longest wavelength of the light in said exit path and witha substantially constant velocity over a major portion of eachreciprocating cycle;

detector means located in the light exit path of said interferometer forgenerating a signal in response to the output in said exit path; and

electronic frequency analyzer means responsive to said signal, formeasuring the amplitude of the various component frequencies making upthe output signal from said detector means. 7. The spectrum analysisapparatus as claimed in claim 6 wherein said means for reciprocatingincludes means for providing a scanning motion at a first velocity and areturn motion at a second velocity, said second velocity being greaterthan said first velocity.

8. The spectrum analysis apparatus as claimed in claim 7 wherein saidmeans for reciprocating provides a second velocity approximately tentimes as great as said first velocity.

9. The spectrum analysis apparatus as claimed in claim 6 wherein saidmeans for continuously reciprocating said mirror comprise a linearelectrodynarnic motor.

10. The spectrum analysis apparatus as claimed in claim 6 wherein saidmeans for reciprocating provides a speed of said reciprocable mirrorduring said constant velocity portion of the reciprocating cycle whichis at least 0.06 mm. per second, providing an AC. signal withoutchopping.

1I.Spectrum analysis apparatus comprising: a Michelson interferometerincluding a continuously reciprocable mirror and having a light exitpath;

means for continuously reciprocating said mirror so that eachreciprocating cycle has (a) an excursion exceeding the longestwavelength of light in said exit path, and (b) a substantially constantvelocity over a major portion of the reciprocating cycle;

detector means located in the light exit path of said interferometer forgenerating a signal in response to the output in said exit path; and

means for frequency analysis responsive to said signal, for measuringthe amplitude of the various com ponent frequencies making up the outputsignal from said detector means.

12. The spectrum analysis apparatus as claimed in claim 11 wherein saidmeans for reciprocating includes means for providing a scanning motionat a first velocity and a return motion at a second velocity, saidsecond velocity being greater than said first velocity.

13. The spectrum analysis apparatus as claimed in claim 12 wherein saidmeans for reciprocating provides a second velocity approximately tentimes greater than said first velocity.

14. The spectrum analysis apparatus as claimed in claim 12 wherein saidmeans for frequency analysis includes blanking means to blank the signalgenerated by said detector means during said return motion.

15. The spectrum analysis apparatus as claimed in claim 11 wherein saidmeans for continuously reciprocating said mirror comprise a linearelectrodynamic motor.

16. The spectrum analysis apparatus as claimed in claim 11 wherein saidmeans for reciprocating provides a speed of said reciprocable mirrorduring said constant velocity portion of the reciprocating cycle whichis at least 0.06 mm. per second, providing an A.C. signal withoutchopping.

17. Spectrum analysis apparatus comprising:

a Michelson interferometer including a continuously reciprocable mirrorand having a light exit path; means comprising a linear electrodynamicmotor for continuously reciprocating said mirror with an excursionexceeding the longest wavelength of the light in said exit path and witha substantially constant velocity of at least 0.06 mm. per second over amajor portion of each reciprocating cycle, each said reciprocating cyclecomprising a scanning motion and a return motion, said scanning motiontaking place at a first velocity and said return motion taking place ata second velocity greater than said first velocity;

detector means located in the light exit path of said interferometer forgenerating a signal in response to the output in said exit path; and

electronic frequency analyzer means responsive to said signal, formeasuring the amplitude of the various component frequencies making upthe output signal from said detector means;

said analyzer means including blanking means to blank the signalgenerated by said detector means during said return motion. 18. For usewith electronic frequency analyzer means [capable of] measuring theamplitude of component frequencies making up an electrical signal,interferometric appartus comprising:

a Michelson interferometer including a continuously reciprocable mirrorand having a light exit path;

means for continuously reciprocating said mirror with an excursionexceeding the longest wavelength of the light in said exit path and witha substantially constant velocity over a major portion of eachreciprocating cycle; and

detector means located in the light exit path of said interferometer forgenerating a signal in response to the output in said exit path, saidsignal being supplied to said electronic frequency analyzer means.

19. The apparatus as claimed in claim 18 wherein said means forreciprocating includes means for providing a scanning motion at a firstvelocity and a return motion at a second velocity, said second velocitybeing greater than said first velocity.

20. The apparatus as claimed in claim 18 wherein said means forcontinuously reciprocating said mirror comprise a linear electrodynamicmotor.

21. The apparatus as claimed in claim 18 wherein said means forreciprocating provides a speed of said reciprocable mirror during saidconstant velocity portion of the reciprocating cycle which is at least0.06 mm. per second, providing an A.C. signal without chopping.

22. The apparatus as claimed in claim 19 wherein said analyzer meansincludes blanking means to blank the signal generated by said detectormeans during said return motion.

References Cited The following references, cited by the Examiner, are ofrecord in the patented file of this patent or the original patent.

UNITED STATES PATENTS 9/1945 D'Agostino et al. 356-106 R OTHERREFERENCES RONALD L. \VIBERT, Primary Examiner Re. 27,9 47 March 26, 197

Patent; No.

Lawrence N. Mertz Inventor(sa) It is ccrtif ism] that: error appears inthe above-:idcnt if I'm! put w: and that said Letters Patent are herebycorrected as shown he] ow:

Column 3, lines 7 through 9, delete "For drivinp' the mirror there isorovided for the mirror and 0011.";

Column 3, line 59, "as" should be --are--;

Signed and sealed this 6th day of August 197 (SEAL) Attest:

C. MARSHALL DANN Commissioner of Patents MCCOY M. GIBSON, JR. AttestingOfficer Disclaimer and Dedication Re. 27,947.Lawrence N. Mertz,Lexington, Mass. INTERFERENCE TECH- NIQUE AND APPARATUS FOR SPECTRUMANALYSIS. Patent dated Mar. 26, 1974. Disclaimer and Dedication filedSept. 8, 1980, by the assignee, Bio-Rad Laboratories, Inc.

Hereby disclaims all claims and dedicates to the Public the entire termof said patent.

[Oflicial Gazette October 12, 1982.]

